1. Field of the Invention
The present invention relates to a transmitter circuit for use in a communications device such as a mobile telephone and a wireless LAN device and more particularly, to a transmitter circuit capable of outputting a transmission signal having high accuracy irrespective of bandwidth and operating with high efficiency, and a communications device using the transmitter circuit.
2. Description of the Background Art
A communications device such as a mobile telephone, a wireless LAN device, or the like has been required to be capable of ensuring accuracy of an outputted signal and operating with low power consumption. And in such a communications device, a transmitter circuit which outputs a transmission signal having high accuracy irrespective of bandwidth and operates with high efficiency is used. Hereinafter, a conventional transmitter circuit will be described.
As the conventional transmitter circuit, for example, there has been a transmitter circuit (hereinafter, referred to as a quadrature modulation circuit) which generates a transmission signal using a method of modulation such as quadrature modulation. Since the quadrature modulation circuit is widely known, description on the quadrature modulation circuit will be omitted. In addition, as a conventional transmitter circuit which has a smaller size and operates with higher efficiency than the quadrature modulation circuit, for example, there has been a transmitter circuit 500 shown in FIG. 18. FIG. 18 is a block diagram illustrating an exemplary configuration of the conventional transmitter circuit 500. In FIG. 18, the conventional transmitter circuit 500 includes a data generator 501, an angle modulator 502, a voltage control section 503, an amplitude modulator 504, a power source terminal 505, and an output terminal 506.
In the conventional transmitter circuit 500, the data generator 501 generates and outputs an amplitude signal and a phase signal. The amplitude signal is inputted to the voltage control section 503. The phase signal is inputted to the angle modulator 502. The voltage control section 503 supplies to the amplitude modulator 504 a voltage in accordance with the inputted amplitude signal. A DC voltage is supplied to the voltage control section 503 from the power source terminal 505. The voltage control section 503 supplies to the amplitude modulator 504, typically, a voltage in proportion to the inputted amplitude signal. As the voltage control section 503, a series regulator or a switching regulator is used.
The angle modulator 502 angle-modulates the inputted phase signal to be outputted as an angle-modulated signal. The angle-modulated signal outputted from the angle modulator 502 is inputted to the amplitude modulator 504. The amplitude modulator 504 amplitude-modulates the angle-modulated signal, while applying the voltage supplied from the voltage control section 503, to be outputted as a modulated signal which has been angle-modulated and amplitude-modulated. This modulated signal is outputted as a transmission signal from the output terminal 506. Such a transmitter circuit 500 is referred to as a polar modulation circuit.
In the conventional transmitter circuit 500, the amplitude signal and the phase signal are separately single-processed in two paths (angle modulator 502 and voltage control section 503) and jointly amplitude-modulated by the amplitude modulator 504. Therefore, it is likely that a difference between a delay time in an amplitude signal path and a delay time in a phase signal path may occur. Hereinafter, the delay time in the amplitude signal path is referred to as the delay time of the amplitude signal, and the delay time in the phase signal path is referred to the delay time of the phase signal. Because such a difference between the delay times is minute, when the transmitter circuit 500 is applied in a modulation method which has comparatively narrow bandwidth, the difference between the delay time of the amplitude signal and the delay time of the phase signal can be disregarded.
In addition, disclosed in the US Published Application No. 2002/141510A1 (hereinafter, referred to as a patent document 1) is a transmitter circuit which is different from the above-mentioned polar modulation circuit and adjusts the difference between the delay times of the amplitude signal and the phase signal which are contained in a transmission signal. FIG. 19 is a block diagram illustrating a configuration of the conventional transmitter circuit 510 disclosed in the patent document 1. In FIG. 19, the conventional transmitter circuit 510 includes a phase modulator circuit 511, an amplifier 512, an amplifier 513, a delay device 514, and a feedback circuit 515.
The phase signal is inputted via the phase modulator circuit 511 to the amplifier 512. The amplitude signal is inputted via the amplifier 513 and the delay device 514 to the amplifier 512. Based on the inputted phase signal and amplitude signal, the amplifier 512 generates a transmission signal. The transmission signal is inputted to the feedback circuit 515. Based on the inputted transmission signal, the feedback circuit 515 detects a difference between a delay time of the amplitude signal and a delay time of the phase signal.
FIG. 10A, FIG. 20C, and FIG. 20C are diagrams explaining a method for detecting the difference between the delay times by means of the feedback circuit 515. FIG. 20A is a diagram showing a change in the transmission signal by using an in-phase and a quadrature-phase signal in a case where the delay times match. FIG. 20B is a diagram showing a time change in an envelope of the transmission signal in a case of FIG. 20A. FIG. 20C is a diagram showing a time change in a phase of the transmission signal in the case of FIG. 20A. In FIG. 20A, FIG. 20B, FIG. 20C, if the delay times between the amplitude signal and phase signal match, time at which the envelope of the transmission signal is minimum and time at which the phase largely changes should match. Conversely, if there is a difference between the delay times of the amplitude signal and the phase signal, there should be a difference between the time at which the envelope of the transmission signal is minimum and the time at which the phase largely changes. Based on this difference between the time at which the envelope of the transmission signal is minimum and the time at which the phase largely changes, the feedback circuit 2205 is capable of detecting the difference between the delay times of the amplitude signal and the phase signal.
Specifically, the envelope detector 5151 detects an envelope of the transmission signal. A minimum detector 512 detects time at which the envelope of the transmission signal is minimum. A phase jump detector 5153 detects time at which a phase of the transmission signal largely changes. When there is a difference between the time at which the envelope of the transmission signal is minimum and the time at which the phase of the transmission signal largely changes, an adjusting circuit 5154 determines that there is the difference between the delay time of the amplitude signal and the delay time of the phase signal. When the adjusting circuit 5154 determines that there is the difference between the delay time of the amplitude signal and the delay time of the phase signal, the adjusting circuit 5154 adjusts timing of outputting the amplitude signal at the delay device 514, thereby matching the delay times of the amplitude signal and the phase signal. Thus, the conventional transmitter circuit 510 detects the difference between the time at which the envelope of the transmission signal is minimum and the time at which the phase of the transmission signal largely changes, thereby adjusting the delay times of the amplitude signal and the phase signal.
And also in Japanese translation of PCT international application No. 2002-530992 (hereinafter, referred to as a patent document 2), disclosed is a detecting circuit for detecting a difference between delay times of an amplitude signal and a phase signal which are contained in a transmission signal. Since the detecting circuit in the patent document detects the difference between the delay times by using a similar method of the above-mentioned feedback circuit 515, detailed description will be omitted.
However, it is likely that when the conventional transmitter circuit 500 (FIG. 18) is applied in a modulation method which has wide signal bandwidth, the difference between the delay times of the amplitude signal and the phase signal contained in the transmission signal may greatly influence a spectrum and a modulation accuracy of the transmission signal. Therefore, the conventional transmitter circuit 500 has a problem of a difficulty in an application thereof to the modulation method which has the wide bandwidth.
Also in the conventional transmitter circuit 510 (refer to FIG. 19) in which the difference between the delay times of the amplitude signal and the phase signal contained in the transmission signal is adjusted, the delay times of the amplitude signal and the phase signal cannot be accurately adjusted. The reason is that when there is a difference between the delay times of the amplitude signal and the phase signal contained in the transmission signal, there occurs distortion in the envelope of the transmission signal. FIG. 21 is a diagram showing a time change in the envelope of the transmission signal in which distortion has occurred. As shown in FIG. 21, when the envelope of the transmission signal is distorted, the conventional transmitter circuit 510 cannot accurately detect time at which the envelope of the transmission signal is minimum and cannot accurately adjust the delay times of the amplitude signal and the phase signal.
And in the conventional transmitter circuit 510, respective sections (i.e., an envelope detector 5151, a minimum detector 5152, a phase jump detector 5153, and an adjusting circuit 5154) of the feedback circuit 515 are analog components. Therefore, the conventional transmitter circuit 510 has problems of a large circuit scale of the feedback circuit 515 and of large power consumption in the feedback circuit 515. Further, when manufacturing the transmitter circuit 510, because the feedback circuit 515 includes the analog components, there arises a problem that precision of the feedback circuit 515 fluctuates.